1. Field of the Invention
The present invention relates to a method of winding a toroidal line filter employed in a primary AC power source terminal of a switching mode power supply (SMPS) for use in supplying power to a printer or other electric or electronic appliance so as to improve an electromagnetic interference (EMI) characteristic.
2. Description of the Related Art
An SMPS is a module type power supply for transforming externally supplied electricity to be suitable for various electric or electronic appliances including computers, TV's, VCR's, wireless communication appliances, or the like. This serves to intermittently control a high frequency which is higher than a commercial frequency by using a switching feature of a semiconductor and to prevent damage. Such SMPS's are becoming more commonly used as a main power supply for electric or electronic appliances because they are advantageous due to their small size and superior efficiency.
Although such SMPS's are advantageous due to their small size and light weight, they have a problem of producing noise and EMI caused by a switching transistor or the like.
A toroidal line filter is employed in a primary AC power supply terminal so as to improve noise and EMI characteristics produced in an SMPS, a typical example of which is shown in FIGS. 1A and 1B.
As shown in FIGS. 1A and 1B, a toroidal line filter comprises a base plate 10, an annular core 20 supported on the base plate 10 and coils 30 wound around the annular core 20.
The coils 30 are wound around the core 20 in such a way that a first coil is wound over several turns upwardly from the lowermost end 20a to the uppermost end 20b of the core 20 (see solid line arrow in FIG. 1B), a second coil is wound downwardly from the uppermost end 20b to the lowermost end 20a of the core 20 (see one-dot-chain line arrow in FIG. 1B), and a third coil is wound upwardly again from the lowermost end 20a to the uppermost end 20b of the core 20 (see two-dot-chain line arrow in FIG. 1B).
In this way, the coils 30 are wound upwardly and downwardly over the entire area of the core 20 three or more times. A toroidal line filter configured in this manner is mounted on a substrate (not shown) by means of plural contacts 11 and 12 projecting downward from the base plate 10.
Such a line filter basically exhibits an inductor function as an L device. That is, because the line filter has a fixed inductance L value, it removes a noise frequency produced from each noise source by counterbalancing the noise frequency with an inductance frequency component of the line filter. Such a capability of the line filter is determined by the permeability of the core 20 and the number of windings of the coils 30. In an ordinary line filter, coil 30 is wound several times upwardly and downwardly over the entire area of the core 20 and thus the current flowing direction is fixed over the entire area of the core 20.
With the design of existing line filters, the number of cores 20 is usually limited to one, two or three because of mass production considerations and the number of windings 30 of the coils 20 is limited because it is necessary for the coils 20 to retain a predetermined thickness due to thermal constraints. If a problem is caused in a certain frequency range from the viewpoint of EMI and thus it is required to provide an improvement for the corresponding frequency band, there is no other way than to add an inductor and a capacitor in order to obtain the desired change in the frequency characteristic with the existing line filters. However, the addition of such components is contrary to the weight-reduction and miniaturization of a product and includes ancillary problems such as increased cost and energy loss.